An MTA employee died early Tuesday morning after falling while working on the subway tracks in Harlem, agency officials said.

The 23-year-old male was part of a work crew that was removing redundant rail in the tunnels near the 125th Street 4/5/6 station at Lexington Avenue, New York City Transit President Andy Byford said.

We’re sad to report that a New York City Transit employee died this morning after sustaining a fall on the 4, 5 and 6 lines near the 125 St station. Service is suspended on the 6 line from 86 St to 3 Av-138, and southbound on the 5 line from Nereid Av to E 180 St. (1/2)

Just before 5 a.m., the worker, who was later identified by a transit union source as St. Clair Ziare Richards-Stephens of the Bronx, fell about 9 feet, “hit the ground and was immediately knocked unconscious,” Byford said, adding that investigators have yet to determine what prompted his fall. Richards-Stephens had been employed with the MTA for six months.

Byford said that responders tried to resuscitate Richards-Stephens for about 45 minutes, but he was pronounced dead at the scene.

“My top priority is to visit his family,” Byford said. “These are the days you dread.”

"This is a tragedy, not just for this young man and his family, but for the entire city,” the Transport Workers Union said in a statement. “While millions of New Yorkers were asleep, this young transit worker was on the job in a tunnel beneath Manhattan so others could get to work or school or wherever else they need to go in the morning. We will be involved in a full investigation to determine how this tragedy happened. Our entire brotherhood and sisterhood of transit workers extend our condolences to his family."

The investigation into Richards-Stephens' death suspended service on the 6 train and caused 4 and 5 trains to run local in both directions between 125th Street and Grand Central-42nd Street, causing delays on those green-line trains as well as on 1, 2 and 3 trains throughout the morning on Tuesday.